Smith's Cloud 
Enormous galaxies like our own, the Milky Way, grew so large by swallowing other galaxies, star clusters, and gas clouds. In fact, this process is underway today in the constellation Aquila, the eagle. Aquila's home to the bright star Altair, a mere 17 light-years away, which is high in the east as night falls.
Far beyond Altair, about 40,000 light-years from Earth, is a cloud of gas that astronomer Gail Smith discovered back in 1963. She found it because the gas is made mostly of hydrogen atoms, which emit radio waves that were detected by radio telescopes.
Astronomers recently studied Smith's Cloud and concluded that it likely has a bright future -- literally.
The cloud is a million times more massive than the Sun, and it's heading toward the star- and gas-filled disk of the Milky Way. In 20 million to 40 million years, the cloud will smash into the outer part of the disk.
The collision should trigger the birth of new stars, as gas in the cloud gets squeezed and collapses under its own weight.
Compared with the overall Milky Way, Smith's Cloud may seem modest -- it's a mere one-millionth as massive. But gas is the raw material for new stars. Many other galaxies have run out of gas and thus no longer give birth to new stars. In contrast, infalling gas like Smith's Cloud ensures that the Milky Way will continue to give birth to new stars far into the future.
Tomorrow: an evening planet.
Script by Ken Croswell, Copyright 2011